This invention generally relates to methods and apparatus for ink jet printing and plotting but more particularly this invention relates to the field of high resolution ink jet color printing and plotting.
During the last decade the use of electrically controllable ink jets in recording devices such as industrial markers, printers, and color hardcopy output devices for computers, has increased measurably. Two principally different methods have been developed: drop on demand methods and methods using continuous ink jets. Both methods have been used both for the printing of alphanumeric characters and to generate computer generated charts and images, often generated in color. Also ink jets have been applied in the field of facsimile transmission.
As a result of intensive research on methods for ink jet printing, the quality of the print out has been improved appreciably during the last few years. The fact of improved quality of print out is especially true for computer controlled ink jet plotters generating color pictures on paper or transparencies, where the ultimate goal is to have an image quality essentially equal to the image quality of a good photographic color print. However, the effort toward achieving this ultimate goal is limited due to the fact that the drop-on-demand method and most of the continuous jet methods are essentially digital devices, i.e., in each picture element (pixel) of the picture they place a drop of ink or no ink at all.
These plotters usually use only four colors of ink (magenta, yellow, cyan, and black). Consequently only a very limited range of color shades can be printed by different combinations of these colors in each pixel. This limitation has been circumvented by the so called dither techniques described by Jarvis, Judice and Ninke in "Computer Graphics and Image Processing", No. 5, 1976, pp 13-40. In the Jarvis, et al article, the picture is divided into a large number of square matrices, each matrix containing a certain number of pixels. A typical matrix size is 4.times.4 or 8.times.8 pixels, i.e., 4 pixels in each row and 4 pixels in each column. Thus each pixel has an area of 1/16 the area of the matrix. Different shades of color can be obtained by filling different numbers of pixels in each matrix with ink. Hence, using a 4.times.4 matrix 16 different shades of a color and white (no color) can be generated, while an 8.times.8 matrix allows the rendition of 64 shades of color in addition to white. There has been described several ways of generating color shades by the matrix method above, e.g. ordered dither, digital half tone, and special unordered dither algorithms. However, due to the relatively large size of the ink drops, all of these methods result in coarse images, images so coarse that the eye can clearly discern the repetitive pattern of the matrices or at the least, the eye perceives a strong and very apparent graininess in the generated color print.
The image quality of the matrix method can be improved appreciably if the color density in each pixel can be varied continuously. If the pixels which make up the matrix are smaller than 0.1.times.0.1 millimeters, then the unaided human eye can no longer resolve the pixels at a normal viewing distance of 20 centimeters. Therefore, an image made up by 8 to 10 of such continuous density pixels to the millimeter has the same appearance, when viewed by the unaided eye, as a truly continuous tone picture, typified by a high grade photographic color print. The fact, of pixels smaller than 0.1.times.0.1 mm appearing as continuous, is used in conventional high quality color printing using offset or gravure plates, where pixel densities of 6 to 8 pixels per millimeter (160 to 200 per inch) are used and these methods generate color shades by varying the size of each pixel point, i.e., the amount of ink applied to each pixel.
The primary object of the present invention is to essentially apply the method used in offset and gravure printing to ink jet printing resulting in dramatic improvements in image quality and resolution of ink jet color prints.